1. Field of the Invention
The present invention relates to a brake booster, and, more particularly, to a tandem brake booster.
2. Background of the Discussion
In a conventional tandem brake booster 10 which has been disclosed in Japanese Utility Model Application (publication no. 58-106265) illustrated in FIG. 1, a first passage 11 for communication between first and second variable pressure chambers 12, 13 is formed by a hole 14 provided on a cylindrical hub 15 of a front movable wall 16 and by hole 17 provided on a cylindrical hub 18 of a rear movable wall 19. A second passage 20 for communication between first and second constant pressure chambers 21, 22 is formed by a hole 23 provided on a case 24 assembled within both cylindrical hubs 15, 18 and by recesses 25, 26 provided on the cylindrical hub 15. In this tandem brake booster 10, inner peripheral edge portions of each diaphragm 27, 28 at both movable walls 16, 19 are utilized as a seal member for both passages 11, 20.
Movable walls 16, 19 are connected by the case 24 to each other and the forward actuating force operating on both movable walls 16, 19 is acted on an output push rod 29 through a reaction rubber disc 30 assembled within a stepped portion 31 of the case 24 and the reaction acts on a valve plunger 32 slidably inserted into a small diameter portion of the case through the reaction rubber disc 30.
A valve mechanism 33 is provided for generating a pressure difference between both constant pressure chambers 21, 22 and both variable pressure chambers 12, 13 by the valve plunger 32, a cylindrical valve 34 and a valve seat 35 provided on the cylindrical hub 18. The amount of sliding of the valve plunger 32 with respect to the cylindrical hub 18 is limited by a key 36 which is inserted into radial holes 37, 38 provided on the cylindrical hub 18 and on the case 24 and is engaged with a small diameter portion of the valve plunger 32 at a fork shaped inner end portion thereof. The axial extent of the radial holes 37, 38 is greater than the thickness of the key 36, so that the key 36 can be moved in a forward and rearward direction by a predetermined amount with respect to cylindrical hub 18 and the case 24.
The key 36 is provided with a contacting portion 39 which is engageable with a housing 46. The contacting portion 39 of the key 36 is contacted with the housing 20 and rearward movement of the valve plunger 32 with respect to the housing 46 is stopped as shown in FIG. 1 which illustrates a non-actuating state of the booster apparatus 10. In addition, further rearward movement of both movable walls 16, 19 is also stopped by contact of a front wall of the radial hole 37 with a front face of the key 36. A rear end of the valve plunger 32 is contacted with the cylindrical valve 34 and cylindrical valve 34 is lifted from the valve seat 35 by a slight amount.
The vacuum transmitted to both constant pressure chambers 21, 22 from a connector 40 mounted on the housing 20 is also transmitted to both variable pressure chambers 12, 13 through a hole 41 provided on the cylindrical hub 18, a clearance between the cylindrical valve 34 and the valve seat 35, and a hole 42 provided on the cylindrical hub 18, so that the pressure difference between constant pressure chambers 21, 22 and variable pressure chambers 12, 13 is eliminated.
As mentioned above, the cylindrical valve 34 is only slightly lifted from the valve seat 35 in the non-actuating state. Therefore, a small forward stroke of an input push rod 43 is required for seating the cylindrical valve 34 on the valve seat 35 and releasing the rear end of the valve plunger 32 from the cylindrical valve 34 to thereby communicate the atmosphere into both variable pressure chambers 12, 13 in the actuating state. The required pressure difference between constant pressure chambers 21, 22 and variable pressure chambers 12, 13 is rapidly generated and the movable walls 16, 19 are also rapidly forwardly moved. The contacting portion 39 of the key 36 is released from the housing 46 by the forward movement of the movable walls 16, 19 during actuation, so that the valve plunger 32 is contacted with the key 36 by the release of the input push rod 43 and is then rearwardly moved with the key 36 to the cylindrical hub 18, until the rear face of the key 36 is contacted with a rear wall of the radial hole 37. Further, the rear end of the valve plunger 32 is contacted with the cylindrical valve 34 and the cylindrical valve 34 is lifted from the valve seat 35 by a relatively large amount. Consequently, the atmosphere within variable pressure chambers 12, 13 quickly flows into constant pressure chambers 21, 22 and the pressure difference between constant pressure chambers 21, 22 and variable pressure chambers 12, 13 is rapidly decreased, and then movable walls 16, 19 are quickly and rearwardly moved.
In the situation where rearward movement of movable walls 16 and 19 occurs, before a part 39 of key 36 contacts with the housing, the valve plunger 32 and the key 36 are rearwardly moved as one by the distance of the movable walls 16 and 19 under the condition where the gap between the front surface of key 36 and the front surface of radial hole 37 exists due to the rear surface of key 36 being connected with the rear surface of radial hole 37.
At such time, the gap between seat 35 and valve 34 is the maximum distance.
When the part 39 of key 36 is contacted with housing, rearward movements of key 36, valve plunger 32 and valve 34 are stopped. However, the movable walls 16 and 19 are further rearwardly moved until the front surface of radial hole 37 is contacted with the front surface of key 36. The extent of the gap formed between seat 35 and valve 34 is decreased and restored to its original infinitely small value by the rearward movement of movable walls 16 and 19 after the part 39 of key 36 is contacted with the housing.
In the above-mentioned booster apparatus, the case 24 is fixedly inserted into both cylindrical hubs 15, 18 and is prevented from being dislodged by a clip 44 fixedly connected at the rear end thereof and is fixed by a nut 45 screwed at the top end thereof and which integrally connects both cylindrical hubs 15, 18. Therefore, such increase of the required number of parts and the corresponding assemblying process, and is of course expensive.